1. Field of the Invention
The invention relates to improvements to bonding conductors, for making connections providing electrical continuity between mechanical parts, particularly in the aeronautical field, and in particular for the purpose of improving the safety of aircraft in flight in stormy conditions.
2. Description of the Prior Art
In aeronautics, it is known to use conductors called "bonding" conductors in order to make, between mechanical parts connected to each other by articulations or links which totally or partially isolate them electrically, links or connections providing electrical continuity between those mechanical parts in order to guarantee their protection from attacks due to lightning currents or to accumulations of electrostatic charges.
Such bonding conductors are used for example between the various members of a rotor head of a rotary-wing aircraft in order to provide electrical continuity between its members and to carry, when the case arises, lightning currents from the impact zones, or the accumulated electrostatic charges, to an outlet point which is in electrical continuity with the structure of the rotary-wing aircraft. In particular, such a bonding conductor can connect each blade of a rotor with the hub of the rotor or, when each blade is connected to the hub by a connecting member, of the type called a bush because its central part is substantially tubular, which is itself connected to the hub by retaining and articulation means in particular comprising an electrically conductive strap attached to the hub, the bonding conductor can be attached, by one of its ends, to this strap, in electrical continuity with the hub and, by its other end, to the bush, in the vicinity of the means by which the corresponding blade is mounted, possibly in a pivoting manner, on this bush.
In these applications, a bonding conductor is subjected to large centrifugal tensile forces due to the rotation of the rotor, alternating flexion and torsional forces due to the alternating movements which it carries out in order to follow the angular deflections in pitch, flapping and drag of the blades with respect to the hub, and repellent forces of magnetic origin, resulting from antagonistic components of the current vector flowing through the bonding conductor when it forms a loop and when it is traversed by a lightning current, these repellent forces becoming greater as the intensity of the current increases and as the radius of curvature of the conductor becomes more pronounced.
In such applications, the use of bonding conductors consisting of a flexible steel cable core surrounded by a braid of electrically conductive wires which is itself covered by a synthetic protective cover, the ends of such conductors being crimped inside conductive connecting lugs, has proved to be unsatisfactory in particular because of the poor mechanical strength of the crimping of the cable surrounded by the braid inside the lugs, when subjected to the effect of the said forces due to centrifugal tension, flexion and torsion and repulsion.
In order to overcome this disadvantage, FR-A-2 679 074 proposes a bonding conductor in which the mechanical strength functions and the electrical functions are partially separated.
In order to constitute a flexible connection device intended to be attached such that each one of its two ends is attached to the respective one of two external elements, and to be subjected, on the one hand, to mechanical movements and stresses due to displacements, possibly relative ones, of the external elements and, on the other hand, to the passages of electrical currents and of electrostatic discharges between the external elements, the bonding conductor according to FR-A-2 679 074 is of the type comprising:
an electrically conductive flexible metal cable, each of whose two ends is attached to the respective one of two electrically conductive metal end-pieces, such that the cable and the end-pieces provide the essential part of the mechanical strength of the bonding conductor between its attachment ends and allow the possible passing of a portion of an electrical current or of an electrostatic discharge, PA1 two electrically conductive metal lugs of high mechanical strength, each one comprising, on the one hand, a part having a housing whose shape is adapted for receiving and retaining an end-piece and the corresponding end of the cable, in such a way as to ensure the transmission of longitudinal forces between the said lug and the said cable, and, on the other hand, a part for attachment to one of the external elements in order to provide simultaneously the mechanical continuity and the electrical continuity of the bonding conductor with the said external element, PA1 at least one assembly of electrically conductive filaments, around the said cable and in electrical continuity with the said lugs and providing the major portion of the electrical connection between the said lugs, and PA1 flexible means of protecting the conductive filaments.
In FR-A-2 679 074, the assembly of conductive filaments is arranged as a braid of metal filaments, and the flexible means of protection are an external protective cover disposed around the braid.
But, furthermore, in order to provide good mechanical strength and good electrical continuity to the connection between each lug, on the one hand, and, on the other hand, an end-piece, the corresponding end of the cable and the braid, the latter does not extend as far as covering the ends of the cable and the end-pieces crimped on these ends, and it is not engaged in the lugs, but, on the contrary, surrounds the housing portion of each lug, whilst itself being surrounded, at the level of this part, with a cylindrical metal ring, which is itself covered by the external protective cover which is disposed around the braid. At each end of the flexible cable, the assembly of this end, the corresponding end-piece, the corresponding lug, the conductive braid and the corresponding ring is joined together by crimping the ring around the braid and the housing part of the lug, or, as a variant, by soldering or brazing using an added metal in the melted state, for example tin, which is poured into the housing part of the lug and around the latter, into the ring and into the portion of the braid disposed inside the ring between the latter and the lug, such that the added metal provides the connection between the previously mentioned various elements of this assembly.
In use it has proved that such a construction does not have sufficient reliability. This disadvantage is related in particular to the possible slipping of the protective external cover with respect to the lugs, to the fatigue fracture of the conductive braid at the level of the junction between the flexible cable and the end of the housing part of the lugs, to the degradation of the flexible metal cable where it emerges from each lug and to the degradation of the conductive braid during the operation of crimping the ring, when the mechanical and electrical connections between the braid and each lug are provided by crimping the corresponding ring.
In general, the disadvantages of this known construction arise because the stresses due to functioning (under the effect of flexion and/or torsional forces, centrifugal forces in flight and repellent forces of magnetic origin in the case of lightning strike), and the stresses due to the crimping, where applicable, are concentrated in the same section of the bonding conductor.